The artificial capillary culture technique provides a pericellular microenvironment that closely resembles the in vivo state. This may allow normal cells to maintain their differentiated functions for prolonged periods of time in vitro. Cells are maintained in a physiologic state without many of the artifactual conditions imposed by more standard methods of tissue culture. The technique is therefore especially well suited for the culture of cells of endocrine origin. Response of the cells to various physiologic agents are being studied as well as the secretion of various hormones over long periods with the possible application to continuous production of hormones for laboratry and clinical uses. The culture unit has been modified by braiding two separate capillary bundles together enabling one bundle to be operated at a higher pressure than the other. This induces a gentle bulk flow between bundles that simulates lymphatic drainage of in vivo tissues. Measurement of labeled compounds of various molecular weights thus passed through solid cell masses grown between the capillaries; mathematical analysis of these data showed that both the convection and diffusion modes of mass transport are important within solid tumors.